The ganzfeld electroretinogram has revealed that patients with chronic Parkinson's disease have a diminished sensitivity of their retina to light; this defect can, in part, be restored by an i.v. infusion of L-dopa. Together, these findings indicate that the retina is another CNS structure that is involved in Parkinson's disease. In addition, morphine modulates the sensitivity of the retina to light. In combination with other studies, we have found that the actions or morphine may be related to the action of dopamine within the retina.